Colleges and universities display substantial differences in the ratio of students to faculty across fields or disciplines. At Harvard University, for example, economics has about 16 students majoring in the subject per full-time-teaching equivalent, while in other departments such as astronomy, Slavic, German, and Celtic, the number of teaching faculty exceeds the number of student majors. We begin by presenting some evidence on the extent of the variation in faculty resource allocation by field and the broad changes over the last several decades. We then consider potential economic explanations for these striking patterns. For example, a basic education production function, which seeks to maximize aggregate student learning subject to a faculty salary budget constraint, will require that faculty be allocated across fields so that relative marginal gains in student learning equal relative faculty salaries. Differences across fields in student-faculty ratios could then arise either from differences in the pedagogical technology across fields or variation in relative faculty salaries. Additional university goals, such as research and graduate program productivity, or adjustment costs, as imposed by the tenure system, could also generate variation across fields in student-faculty ratios. However, we have only limited evidence that these arguments can explain the ongoing disparities in student-faculty ratios across fields and disciplines, which suggests that a substantial part of the explanation may reside in the politics rather than the economics of decision making in institutions of higher education.